Calixarenes are complex cyclic compounds that can undergo self-assembly to form guest host complexes. In such complexes, a guest component can be incorporated within the calixarene-based structure which encapsulates the guest material. The simplest calixarene is calix(4)arene in which four phenyl groups are linked together in a cyclic array by methylene bridges which are proximal to the OH groups of the phenols. Stress induced within the calixarene molecules result in indented or bowl-shaped, and the calixarenes can be assembled to form a supramolecular structures.
The assembly of organic molecular crystals such as those based on supramolecular assemblies of calixarene molecules is primarily controlled by a variety of intermolecular interactions which, in unison, immobilize the building blocks to form stable arrays. When these materials are heated beyond their melting or sublimation points, the cohesive forces are overcome, resulting in increased mobility and disorganization of the molecules. The molecules of a solid can also be mobilized by processes such as dissolution and solid-solid phase changes. The latter can occur as a result of physical stimuli (e.g. temperature, pressure, radiation) or the gain or loss of ancillary molecular components. While inclusion of either a liquid or gaseous guest by a solid matrix is a well-known phenomenon, the mechanisms of such processes are not well defined. In organic solid state guest host assemblies where transport of the guest through the host, and subsequent complexation, usually involves concomitant reorganization of the host lattice (7). Guest-induced lattice rearrangement often result in severe fracturing of single crystals into polycrystalline material. When fracturing does not occur, alternative mechanisms postulate the presence of stable channels through which mobile guest molecules diffuse until a thermodynamically stable host-guest structure is achieved.